Variable gain amplifier

ABSTRACT

The variable gain amplifier of the present invention includes at least an operation amplifier. By choosing one of output stages, a feedback resistor is selected and the gain of the variable gain amplifier is decided according to the resistance of the selected feedback resistor, as desired. By adjusting the gain of the variable gain amplifier, the received signals can be amplified or attenuated in accordance with design requirement. The variable gain amplifier can include a two-stage architecture, in which a first stage is used for coarse gain adjustment and a second stage is used for fine gain adjustment. The gain of the two-stage variable gain amplifier can be easily adjusted to a desired value.

1. FIELD OF THE INVENTION

[0001] The present invention relates to an operational amplifier, andmore particularly, to a variable gain amplifier.

2. BACKGROUND OF THE INVENTION

[0002] Please refer to FIG. 1, which is a schematic diagram depictingthe framework of a conventional variable gain amplifier 100. Thevariable gain amplifier 100 has an operational amplifier 110, a resistorset 120 having a plurality of serial-connected resistors Rf1, Rf2, . . ., Rfn, and a switch set 130 having a plurality of switches SW1, SW2, . .. , SWn corresponding to the resistors respectively. The switches aregenerally implemented with MOS transistors. Under such configuration ofFIG. 1, a gain G of the variable gain amplifier 100 is given by:G=1+^(Rf)/_(Rg), wherein Rf is a resistance seen between the output Voand the inverting input (−) of the operational amplifier 110, while Rgis a resistance seen between the inverting input (−) of the operationalamplifier 110 and a ground node Vag. Both the resistance Rf and Rgdepend on the on/off state of the corresponding switches in the switchset 130. The conventional variable gain amplifier 100 can ensure amonotonic variation in gain, and can avoid nonlinear distortion and gainerror caused by the MOS switches since there is no current flowingthrough the MOS switches.

[0003] However, since the signal at the non-inverting input (+) of theoperational amplifier 110 varies with the input signal Vi and the inputdynamic range of the operational amplifier 110 is comparatively small,the conventional variable gain amplifier 100 tends to suffer from moresignificant distortion. In addition, the conventional variable gainamplifier 100 cannot perform signal attenuation and possesses inferiorgain accuracy.

[0004] Please refer to FIG. 2, which is a schematic diagram depictingthe framework of another conventional variable gain amplifier 200. Thevariable gain amplifier 200 has an operational amplifier (OPA) 210, aninput resistor Ri, a feedback resistor set 220 having a plurality ofserial-connected resistors Rk1, Rk2, . . . , Rkn, and a switch set 230having a plurality of switches Sk1, Sk2, . . . , Skn correspondingrespectively to the resistors. The switches are generally implementedwith MOS transistors. Under such configuration of FIG. 2, a gain G ofthe variable gain amplifier 100 is given by: G=^(Rk)/_(Ri), wherein Rkis an equivalent resistance shown by the resistor set 220. As seen inFIG. 2, the non-inverting input (+) of the OPA 210 is fixed to a groundnode Vag. Therefore, the OPA 210 tends to have smaller distortion.

[0005] However, since current will flow through the MOS switches in thisframework, the nonlinearity of the MOS switch may incur signaldistortion. Also, gain error may be induced by the impedance of the MOSswitch. In addition, in FIG. 2 if the tolerable noise level is low, theinput resistor Ri with a small impedance and the feedback resistors withsmall impedances are needed. However, when the impedances of the inputresistor and the feedback resistors are of small values, the MOSswitches with large equivalent resistances is required so as to reducedistortion and gain error. Unfortunately, when large-resistance MOSswitches are adopted, the parasitic capacitors thereof tend to causeloop instability and substrate coupling issues.

SUMMARY OF THE INVENTION

[0006] It is one of the many objects of the present invention to providea variable gain amplifier capable of amplifying and attenuating thereceived signals with the characteristics of low distortion and lownoise.

[0007] According to embodiments of the present invention, a variablegain amplifier is disclosed. The variable gain amplifier comprises aninput resistor coupled to an input signal; an operational amplifiercomprising a pre-drive stage coupled to the input resistor and aplurality of output stages, each of which coupled to the pre-drivestage; and a plurality of feedback resistors, each feedback resistorbeing coupled to the pre-drive stage by one end thereof and coupled toone of the plurality of output stages by another end thereof. A feedbackloop is formed by one of the output stages chosen by a first controlsignal and the feedback resistor corresponding to the chosen outputstage.

[0008] These and other objectives of the claimed invention will no doubtbecome obvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment, which isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a framework of a conventional variable gain amplifier.

[0010]FIG. 2 is a framework of another conventional variable gainamplifier.

[0011]FIG. 3 is a schematic diagram of a variable gain amplifieraccording to an embodiment of the present invention.

[0012]FIG. 4 is a schematic diagram of a variable gain amplifieraccording to another embodiment of the present invention.

[0013]FIG. 4A is a schematic diagram of a variable gain amplifieraccording to yet another embodiment of the present invention.

[0014]FIG. 5 is an embodiment of the switch of FIG. 4.

[0015]FIG. 6 is an embodiment of the operational amplifier of theoperational amplification unit with reference to FIG. 4.

[0016] FIG.7 is an embodiment of the output stage of the operationalamplifier with reference to FIG. 4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] According to embodiments of the present invention, variable gainamplifiers comprising operational amplifiers and resistors aredisclosed. The variable gain amplifier may comprise an input resistor, apre-drive stage of an operational amplifier, a plurality of outputstages of the operational amplifier and a plurality of feedbackresistors each coupled respectively to a corresponding output stage. Inone embodiment of the present invention, the pre-drive stage, the outputstages and the feedback resistors may be lumped on a single integratedcircuit (IC), and the input resistor may either be integrated on thesame IC or be disposed externally. One end of each input resistor isconnected to the input signal, and the other end thereof is connected toan input pad of the IC. In another embodiment, the input resistor may beconnected to the IC by way of an electrostatic discharge (ESD)protection device so as to prevent the damage to the variable gainamplifier from happening while the magnitude of the received signal istoo large.

[0018] In one embodiment of the present invention as seen in FIG. 3, avariable gain amplifier 300 comprises an operational amplifier 310, aninput resistor Ri, and a plurality of feedback resistors Rf1, Rf2, . . ., Rfn. The operational amplifier 310 further comprises a pre-drive stage312, and a plurality of output stages 3141, 3142, . . . , 314 n. Thenon-inverting output of the pre-drive stage 312 is connected to thenon-inverting inputs of the plurality of output stages 3141, 3142, . . ., 314 n, and the inverting output of the pre-drive stage 312 isconnected to the inverting inputs of the plurality of output stages3141, 3142, . . . , 314 n. Each output stage 314 i is coupled to acorresponding feedback resistor Rfi and also is coupled to acorresponding control signal Cti, where i=1, . . . , n. The output ofeach output stage 314 i is coupled to the inverting input of thepre-drive stage 312 via a corresponding feedback resistor Rfi. One endof the input resistor Ri is connected to the input signal Vi, and theother end of the input resistor Ri is connected to an input pad and isfurther connected to the inverting input of the pre-drive stage 312 viaan ESD 330.

[0019] Controlled by the control signals Ct1, Ct2, . . . , Ctn, a gain Gof the operational amplifier 300 is determined by the output stage 314 ichosen and driven by the foregoing control signals accompanying thefeedback resistor Rfi coupled thereto, and is given as: G=^(Rfi)/_(Ri).It is noted that the resistances of the feedback resistors are properlypredetermined according to design requirement so as to provide differentgain values with different output stages being chosen. Thus, the gain Gof the variable gain amplifier 300 can be adjusted as desired and thereceived signal can be amplified/attenuated by employing the control ofthe output stages. In this regard, the phenomena of loop instability andsubstrate coupling, as well as non-linear distortion can be alleviated.Hence, the variable gain amplifier 300 can be utilized in applicationsdemanding very low noise level, such as the asymmetric digitalsubscriber line (ADSL) communication system.

[0020] Please refer to FIG. 4, which is a schematic diagram of avariable gain amplifier 400 according to another embodiment of thepresent invention. The variable gain amplifier 400 comprises a firststage operational amplification unit 410 and a second stage operationalamplification unit 420, in which the first stage operationalamplification unit 410 is used for coarse gain adjustment and the secondstage operational amplification unit 420 is used for fine gainadjustment. By adopting the two-stage gain adjustment approach, sincethe input referred noise of the second stage operational amplificationunit 420 can be far higher than that of the first stage operationalamplification unit 410, both input resistors with larger resistances andfeedback resistors with larger resistances can be adopted, and MOSswitches with smaller resistances can also be used, without causingobvious non-linear distortion and gain error.

[0021] The first stage operational amplification unit 410 as seen inFIG. 4 comprises an operational amplifier 412, a input resistor Ri and aplurality of feedback resistors 4131, 4132, . . . , 413 n. Theoperational amplifier 410 comprises a pre-drive stage 411, and aplurality of output stages. 4131, 4132, . . . ,413 n. Each output stage413 i is controlled by a corresponding control signal Cti, where i=1, .. . ,n. As those skilled in the art can appreciate, the first stageoperational amplification unit 410 in FIG. 4 is similar to the variablegain amplifier 300 in FIG. 3, and detailed description is thus omittedherein for simplicity.

[0022] The second stage operational amplification unit 420 as seen inFIG. 4 comprises an operational amplifier 422, a plurality of inputresistors Rs1, Rs2, . . . , Rsn, a plurality of switches Sw1, Sw2, . . ., Swn corresponding respectively to each input resistor, a plurality offeedback resistors Rk1, Rk2, . . . , Rkn, and a plurality of switchesSk1, Sk2, . . . , Skn corresponding respectively to each feedbackresistor. Each input resistor Rsi is coupled to the inverting input ofthe operational amplifier 422 by way of the corresponding switch Swi,wherein every switch Swi is controlled by a control signal Cti of thefirst stage operation amplification unit 410, wherein i=1, 2, . . . , n.The non-inverting input of the operation amplifier 422 is connected to avirtual ground Vag. The output of the operational amplifier 422 iscoupled to the inverting input thereof by way of the feedback resistorsRk1, Rk2, . . . , Rkn and the corresponding switches Sk1, Sk2, . . . ,Skn.

[0023] In this embodiment, the first stage operational amplificationunit 410 is used for coarse gain adjustment and the second stageoperational amplification unit 420 is used for fine gain adjustment,where the overall operation process is illustrated using the followingexample. Initially, one output stage out of the output stages 4131,4132, . . . , 413 n, that is chosen by the plurality of control signalsCt1, Ct2, . . . , Ctn, is couple to the pre-drive stage 414, forenabling a gain Gc of the first stage amplification unit 410, which isalso the coarse gain of the overall variable gain amplifier, to beGc=^(Rfi)/_(Ri), wherein Rfi is the resistance of the feedback resistorcorresponding to the chosen output stage. It is noted that the frameworkof the second stage amplification unit 420 is similar to that of theaforementioned variable gain amplifier with reference to FIG. 2 and thecorresponding description. Thus, the fine gain Gf of the secondoperational amplification unit 420 is determined by the MOS switch Swicorresponding to the chosen output stage and a selected switch Ski.Finally, the total gain of the variable gain amplifier 400 is G=Gc×Gf.

[0024] The adjustment ranges of both the coarse gain and the fine gaincan be varied as desired. For instance, the adjustment range of thecoarse gain Gc depends, among other factors, on the number of the outputstages 413 i, i=1, . . . ,n and the corresponding feedback resistorsRfi, i=1, . . . ,n. Similarly, the adjustment range of the fine gain Gfdepends, among other factors, on the number the feedback resistors Rki,i=1, . . . ,n and the corresponding Ski, i=1, . . . ,n. Thereby thepresent invention has good design flexibility for fulfilling all kindsof requirement.

[0025] For example, when designing a low-noise variable gain amplifierwith −18 dB˜23 dB gain range having 1 dB step, the variable gain rangeof the first stage amplification unit 410 may be set to be −18 dB˜18 dBwith 6 dB step, and the variable gain range of the second stageamplification unit 420 may be set to be 0 dB ˜5 dB having 1 dB step.Assuming that the first stage input impedance Ri is 1 kΩ, then sevenfeedback resistors Rf1, Rf2, . . . , Rf7 are required in the first stageoperational amplification unit 410 since the variable gain range is −18dB˜18 dB with 6 dB step, wherein resistances of the seven feedbackresistors are 125 Ω, 250Ω, 500Ω, 1 kΩ, 2 kΩ, 4 kΩ, 8 kΩ, respectively.All the second stage input impedance Rs1, Rs2, . . . , Rs7 are then setto be 20 kΩ. Six feedback resistors Rk1, Rk2, . . . , Rk6 are requiredin the second stage operational amplification unit 420 since thevariable gain range is 0 dB˜5 dB with 1 dB step, wherein resistances ofthe six feedback resistors are 20 kΩ, 2 kΩ, 3 kΩ,3 kΩ, 4 kΩ, 4 kΩ.

[0026] Please refer to FIG. 4A, which is a schematic diagram of avariable gain amplifier according to yet another embodiment of thepresent invention. The variable gain amplifier 400 has a first stageoperational amplification unit 410 and a second stage operationalamplification unit 420, which both are basically similar to those ofFIG. 4. The difference between the embodiment of FIG. 4 and theembodiment of FIG. 4A is that the plurality of input resistors Rsi, i=1,. . . ,n, are replaced by a single input resistor Rs. As a result, theadjustment range of the fine gain of the second stage amplification unit420 depends, among other factors, on the number the feedback resistorsRki, i=1, . . . ,n, and the corresponding switches Ski, i=1, . . . ,n.

[0027]FIG. 5 is an embodiment of the switches of FIG. 4. As seen in FIG.5, the switch 510 comprises a transmission gate configuration composedof two MOS transistors. A PMOS transistor and an NMOS transistor arecontrolled to turn on/off by a signal SwiB and a complementary signalSwi thereof, respectively. The switch 510 composed of the two MOStransistors is illustrated as a circuit structure 520. When referencingto the aforementioned embodiments, the signal Swi of the switch 510 isthe respective control signal Cti of the operational amplification unit410.

[0028] Please refer to FIG. 6, which is an embodiment of an operationalamplifier 610 used in the operational amplification unit 410. Theoperational amplifier 610 comprises a pre-drive stage 612 and a chosenoutput stage 614.

[0029] FIG.7 is an embodiment of the output stage 614 of the operationalamplifier 610 illustrated by an output stage 710 and a circuit symbolthereof 720. The output stage 710 comprises two output MOS transistorsMon, Mop, and four MOS switches Msw1, Msw2, Msw3 and Msw4, wherein thetransistor Mon is a N-type transistor and the transistor Mop is a P-typetransistor. The MOS switches Msw1 and Msw4 are controlled by the controlsignal Cti, while the MOS switches Msw2 and Msw3 are controlled by thecomplementary signal CtiB of the control signal Cti.

[0030] When the output stage 710 is chosen, that is, in this embodiment,the control signal Cti being set to high, the MOS switches Msw1 and Msw2are on and the MOS switches Msw3 and Msw4 are off, and the output stage710 is therefore activated.

[0031] While the preferred embodiment of the invention has been setforth for the purpose of disclosure, modifications of the disclosedembodiment of the invention as well as other embodiments thereof mayoccur to those skilled in the art. Accordingly, the appended claims areintended to cover all embodiments which do not depart from the spiritand scope of the invention.

What is claimed is:
 1. A variable gain amplifier comprising: an inputresistor coupled to an input signal; an operational amplifier furthercomprising: a pre-drive stage coupled to the input resistor; and aplurality of output stages, one of the output stages is chosen by acontrol signal to couple to the pre-drive stage; and a plurality offeedback resistors, each feedback resistor being coupled to thepre-drive stage by one end thereof and coupled to one of the pluralityof output stages by another end thereof; wherein a feedback loop isformed by the chosen output stages and the feedback resistorcorresponding to the chosen output stage.
 2. The variable gain amplifierof claim 1, wherein the input resistor is coupled to the operationalamplifier by way of an electrostatic discharge protection device.
 3. Thevariable gain amplifier of claim 1, wherein each of the output stagesfurther comprises: a PMOS transistor, whereof a source is connected to asupply voltage; an NMOS transistor, whereof a source is grounded and adrain is coupled to the PMOS transistor; a first MOS switch coupledbetween the pre-drive stage and a gate of the NMOS transistor, a gate ofthe first MOS switch being coupled to the first control signal; a secondMOS switch coupled between the pre-drive stage and a gate of the PMOStransistor, a gate of the second MOS switch being coupled to a secondcontrol signal; a third MOS switch coupled between the gate of the NMOStransistor and ground, a gate of the third MOS switch being coupled tothe second control signal; and a fourth MOS switch coupled between thegate of the PMOS transistor and the supply voltage, a gate of the fourthMOS switch being coupled to the first control signal; wherein the secondcontrol signal is a complementary signal of the first control signal. 4.The variable gain amplifier of claim 1, wherein each output stagefurther comprises: a PMOS transistor coupled to a supply voltage; a NMOStransistor coupled between the PMOS transistor and ground; a firstswitch; a second switch; wherein the first switch is used for connectinga gate of the PMOS transistor to the pre-drive stage when the firstcontrol signal is in a first state, and the second switch is used forconnecting a gate of the NMOS transistor to the pre-drive stage when thefirst control signal is in the first state.
 5. A variable gain amplifiercomprising: a first stage operational amplification unit for providing afirst gain, comprising: a first input resistor coupled to an inputsignal; an operational amplifier comprising: a pre-drive stage coupledto the input resistor; and a plurality of output stages, each outputstage coupled to the pre-drive stage; and a plurality of first feedbackresistors, each first feedback resistor being coupled to the pre-drivestage by one end thereof and coupled to one of the plurality of outputstages by another end thereof; and a second stage operationalamplification unit coupled to the first stage operational amplificationunit, for providing a second gain: wherein a first feedback loop isformed by one of the output stages chosen by a first control signal andthe first feedback resistor corresponding to the chosen output stage,and a gain of the variable gain amplifier corresponds to the first andthe second gain.
 6. The variable gain amplifier of claim 5, wherein thesecond stage operational amplification unit comprises: a plurality ofsecond input resistors, each second input resistor being coupled to acorresponding first feedback resistor of the first stage operationalamplification unit; a plurality of first switches, each first switchbeing coupled to a corresponding second input resistor; a secondoperational amplifier coupled to the plurality of first switches; aplurality of second feedback resistors, being serial-connected with oneanother, being connected to the plurality of first switches by one endof the serial-connected second feedback resistors and an output of thesecond operational amplifier by another end thereof; and a plurality ofsecond switches, each second switch being coupled to the plurality offirst switches by one end and to a corresponding second feedbackresistor by another end.
 7. The variable gain amplifier of claim 5,wherein the second stage operational amplification unit comprises: aplurality of first switches, each first switch being coupled to acorresponding first feedback resistor; a second input resistor coupledto the plurality of first switches; a second operational amplifiercoupled to the second input resistor; a plurality of second feedbackresistors, being serial-connected with one another, being connected tothe second input resistor by one end of the serial-connected secondfeedback resistors and an output of the second operational amplifier byanother end thereof; and a plurality of second switches, each secondswitch being coupled to the second input resistor by one end and to acorresponding second feedback resistor by another end.
 8. The variablegain amplifier of claim 5, wherein the first input resistor is coupledto the operational amplifier by way of an electrostatic dischargeprotection device.
 9. The variable gain amplifier of claim 5, whereineach of the output stages further comprises: a PMOS transistor, whereofa source is connected to a supply voltage; an NMOS transistor, whereof asource is grounded and a drain is coupled to the PMOS transistor; afirst MOS switch coupled between the pre-drive stage and a gate of theNMOS transistor, a gate of the first MOS switch being coupled to thefirst control signal; a second MOS switch coupled between the pre-drivestage and a gate of the PMOS transistor, a gate of the second MOS switchbeing coupled to a second control signal; a third MOS switch coupledbetween the gate of the NMOS transistor and ground, a gate of the thirdMOS switch being coupled to the second control signal; and a fourth MOSswitch coupled between the gate of the PMOS transistor and the supplyvoltage, a gate of the fourth MOS switch being coupled to the firstcontrol signal; wherein the second control signal is a complementarysignal of the first control signal.
 10. The variable gain amplifier ofclaim 5, wherein each output stage further comprises: a PMOS transistorcoupled to a supply voltage; a NMOS transistor coupled between the PMOStransistor and ground; a first switch; a second switch; wherein thefirst switch is used for connecting a gate of the PMOS transistor to thepre-drive stage when the first control signal is in a first state, andthe second switch is used for connecting a gate of the NMOS transistorto the pre-drive stage when the first control signal is in the firststate.
 11. A variable gain amplifier comprising: a pre-drive stagecomprising a first input node for receiving a input signal; a pluralityof output stages, wherein one of the output stages is selected by acontrol signal to couple to the pre-drive stage; and a plurality offeedback impedance, each of the feedback impedances being coupledbetween the pre-drive stage and one of the output stages; wherein thegain of the variable gain amplifier is determined according to one ofthe feedback impedances coupled to the selected output stages and thepre-drive stage.
 12. The variable gain amplifier of claim 11, whereinthe pre-drive stage comprising a first output node and a second outputnode, each of the output stages comprising a first input node, a secondinput node, and a output node, and each of the feedback impedance iscoupled between the first input node of the pre-drive stage and theoutput node of the corresponding output stage.
 13. The variable gainamplifier of claim 12, wherein the first input node of the selectedoutput stage is coupled to the first output node of the pre-drive stageand the second input node of the selected output stage is coupled to thesecond output node of the pre-drive stage.
 14. The variable gainamplifier of claim 11, wherein the variable gain amplifier furthercomprising an input impedance coupled to the first input node of thepre-drive stage, wherein the gain of the variable gain amplifier isdetermined according to the input impedance and one of the feedbackimpedances coupled to the selected output stages and the pre-drivestage.